1. Field of the Invention
This invention relates to oscillators; and more particularly, to oscillators for use in color television transmitting apparatus.
2. Description of the Prior Art
Existing color television transmitters typically utilize crystal oscillators in a temperature controlled environment as their basic frequency determining elements. For example, transmitters have utilized low frequency (i.e., 100-200 KHz) crystals as the exciter for the audio portion of the television signal. Although the crystal provided the proper frequency, the frequency stability (i.e., drift) was unsatisfactory and led to interference which is noticeable on a television receiver's picture tube. Even though these oscillators are capable of maintaining the output frequencies of the aural and visual transmitters well within the .+-.1,000 Hz required by the Federal Communications Commission Rules, the intercarrier 4.5 MHz separation between the picture carrier frequency and the sound carrier frequency changes throughout the broadcast day as the crystals drift around their nominal frequencies. This change in separation can still be within the Federal Communications Commission specification of .+-.1,000 Hz, but causes a very undesirable interference seen on the picture tube of a television receiver.
This interference is referred to as "920 beat" which refers to the frequency difference between the intercarrier 4.5 MHz frequency and the color subcarrier 3.5 MHz frequency, or approximately 0.920 MHz. The interference is a receiver phenomenon caused by color television transmitters and is recognized by the presence of closely spaced gray lines usually sloping from the upper right to the lower left on television receiver picture tubes. Its presence is the result of the mixing of the two demodulated carriers, the intercarrier and the color subcarrier, at the receiver detector diode which, being a nonlinear device, forms sum and difference frequencies due to the presence of these carriers. The 920 KHz difference is the one of interest, since it falls into the video amplifier pass band.
The 920 beat problem is known and has been discussed in the McGraw-Hill television series, Donald G. Fink, Consulting Editor, 1954, Section 24-12, pages 611-612. The beat between the intercarrier and the color subcarrier at the difference frequency of 920 KHz is objectionable because the relatively low beat frequency produces a coarse interference pattern in the reproduced picture. Experiments have shown that this beat interference is much less objectionable if its frequency is an odd multiple of one-half of the horizontal line frequency because of the resultant interlace. Since a frame of 525 lines consists of two interlaced fields each of 2621/2 lines, if the interference is opposing in successive fields, the resultant frame has a reduced interference in comparison to either of its component fields.
In order to provide the desired interlacing for frequencies resulting from the beat between the color subcarrier and the intercarrier, the required 4.5 MHz separation is defined as the 286th harmonic of the horizontal line frequency. This is an even harmonic, resulting in no frequency interlace for 4.5 MHz, but since this beat usually has low amplitude in the video circuits, the fine pattern is not objectionable in the reproduced picture. Making 4.5 MHz an even harmonic with the color subcarrier an odd harmonic automatically makes the beat frequency of 0.920 MHz between the color subcarrier and the intercarrier an odd multiple of one-half the line frequency and results in reduction of the 920 (0.920 MHz) beat interference by interlace.
Since 4.5 MHz is specifically the 286th harmonic of the line frequency, the horizontal line scanning frequency becomes: ##EQU1##
With 525 horizontal lines per frame and 2621/2 lines per field, the vertical field scanning frequency becomes: ##EQU2##
The color subcarrier frequency is chosen as the 455th harmonic of one-half the horizontal line scanning frequency, which becomes: ##EQU3##
It can be readily seen that preceision frequency control of the 4.5 MHz intercarrier separation is essential to a color television system, since all of the other requisite frequencies are derived from it. The intercarrier frequency stability must be held as closely as that of the color subcarrier, within .+-.10 Hz and a rate of change not to exceed 0.1 Hz per second. This is virtually impossible when two transmitter crystals, one for the picture carrier and one for the sound carrier, are employed as frequency control devices. The 4.5 MHz separation varies from minute to minute as the crystal heaters cycle off and on and the crystal temperatures change. Typically, the 920 beat has objectionable periods through the day. Such as, for example, periods when the beat frequency varies from being an odd multiple of one-half of the horizontal line frequency. Additionally, variation of the 920 beat frequency is typically more noticeable than a fixed variation of the 920 beat frequency from the desired frequency, much as a moving light is more noticeable than a steady light displaced from a desired location. One possible method to maintain both the aural and visual transmitter frequencies to such close tolerances uses a rubidium standard, but the cost is prohibitive for many broadcasters.